Export: Allow install tree inside build tree, but disallow inside src tree.

Disallow other combinations too.
Merge remote-tracking branch 'origin/master' into install-prefix-in-interface

Conflicts:
	Tests/RunCMake/RunCMake.cmake
Merge topic 'install-prefix-in-interface' into next

daf3c291 Merge remote-tracking branch 'origin/master' into install-prefix-in-interface
63873f70 Export: Allow install tree inside build tree, but disallow inside src tree.